The size and weight of small, unobtrusive devices is dominated by discrete electrical components mounted to the internal circuit boards. In these cases, the area on the surfaces of the circuit board is very valuable for adding components but as components are added, the height and mass of the completed circuit must increase. Semiconductors are the basic building blocks used to fabricate the wide variety of electronic systems used in commercial, aerospace and defense applications. Presently, every semiconductor device or combination thereof is first mounted on an independent chip carrier, then wire bonds are attached from the contact pads of the semiconductor bare-die to the leads in the chip carrier, and finally the whole assembly is encapsulated in order to package and protect the delicate bare-die and its wire bonds. The resulting assembly on a printed circuit board is rather bulky compared with the bare-die itself and thus limits the level of miniaturization that can be achieved in a circuit requiring several packaged semiconductor devices and associated passive components. Currently there are no techniques capable of transferring, mounting and interconnecting unpackaged semiconductor devices (bare-dies) onto circuit boards or other types of substrates. Most of the steps required for the manufacture of an electronic circuit on a traditional printed circuit board rely on pick-and-place machinery which is designed to handle the packaged devices and requires their placement on the surface of the board and not inside the substrate. Techniques for embedding passive components such as resistors and capacitors have been developed to address miniaturization goals, albeit partially, since they are not capable of embedding active components. These embedding techniques rely on the addition of thick polymer film compositions on a 2-D geometry, effectively building a laminated circuit board from the bottom up, and requiring subsequent tuning in order to achieve the required resistance or capacitance values. Such approaches are inherently limited as they require development of each individual circuit design, require significant volume for the final circuit design, do not allow the fabrication of truly embedded circuits, reduce the ability to add additional components or modify designs to circuits and are not compatible with active components such as transistors, LED's and integrated circuits.
Microelectronic circuits traditionally are fabricated by placing packaged components, such as integrated circuits and surface-mount devices, on the surface of a circuit board and connecting the components together. As the demand for miniaturization has grown, however, the size of these packaged components has become a limiting factor.
There are various ways to address this limitation; for instance, application-specific integrated circuits, which eliminate the need for individual components. Instead, the entire circuit is fabricated in one fell swoop and then packaged. However, development of these single-chip designs is costly and time-consuming, and once the chips are completed, their designs are very difficult to change.
A better solution to achieving desired component densities is to place unpackaged components, such as unpackaged active semiconductor integrated circuits or bare dies, inside the circuit board. These electronic circuits, comprising the embedded components and their interconnects, promise to advance the miniaturization of electronics manufacturing. Embedding components allows a significant reduction of the weight and volume of a circuit. It also leads to shorter interconnects and reduced parasitic inductance, thereby enhancing electrical performance.
Embedding passive components, such as resistors and capacitors, inside a circuit board is not a new technology. However, embedding active semiconductor components has been demonstrated only in specialized applications because the bare semiconductor die are very fragile and tend to be damaged easily by the robotic systems (called “pick-and-place” tools) that mount the devices on a circuit board. Furthermore, pick-and-place tools are ineffective at handling small (less than 1 mm square) or extremely thin (less than 50 μm thick) die, and they are unsuitable also for high-throughput applications (more than 10 devices per second). Avoiding these limitations requires new approaches to the assembly of embedded bare die.
The use of laser-based transfer processes for the release and assembly of structures or devices was initially studied by Holmes et al. In their work, the authors demonstrated for the first time, (see Holmes et al, J. Microelectromech. Syst., 7, (1998) 416, and Holmes, et al., Proc. 19th Int. Congress Applications of Lasers & Electro-Optics (ICALEO), Dearborn, USA, (2000) p.D-1). Holmes disclosed the use of laser-transfer processes for the batch assembly of MEMS structures. Despite the success obtained with MEMS structures, this group did not attempt the application of this process to the transfer of functional semiconductor bare die. It is important to distinguish the laser-based transfer process from the well established laser lift-off technique, which can be used to release functional bare die devices such as individual GaN pn-diodes without degrading their performance. In laser lift-off, the devices are separated from a bonded structure, due to the laser-induced melting of an intermediate layer. However, in this technique, the devices are never transferred across a gap. More recently, Karlitskaya et al., SPIE Proc. 5448, (2004) 935, proposed a thermal model for the laser-release process of relatively small Si die (200 μm×200 μm). Their model showed that the release threshold is below the thermal damage threshold for the reverse side of the die (<673 K), based on heat diffusion of the absorbed laser pulse through the Si substrate. In their work, the backside of the die was exposed to the laser pulse, thus protecting the front patterned side from damage due to excessive heating. However, the authors did not discuss whether or not the dies were damaged after the transfer. Furthermore, this configuration is not very practical for embedding individual devices, due to the difficulties in establishing the electrical connections to the pads on the patterned side of the die, which end up facing the bottom of the pocket in which the device has been embedded. A better approach is to transfer the die with its patterned or active side facing up, enabling direct-write approaches to print the electrical interconnects to each pad on the die. The challenge, however, is to be able to illuminate the active region of the die with the laser release/transfer pulse without damaging it.